Normally a conventional rig up will be comprised of stacked up heave eliminators, which comprises means for keeping the tension in a riser with the movement of a floating vessel, surface flow tree (SFT), equipment for performing wire line or coiled tubing operations into the well, and a surface blow out preventer (SBOP) on the rig floor as part of the conventional work over riser. There will in some instances also be arranged a telescopic element in the riser below the SBOP. For performing wire line or coiled tubing operations the riser string will normally be depressurized and the rig heave motions vs. the workover riser string are compensated by keeping the upper end of the riser string with the SBOP in relative position in relation to the vessel. In such a configuration the upper part of the telescopic element the adapter, SBOP and eventual coiled tubing equipment or wire line equipment will be lifted in a tension frame and moved with the necessary relative movement in relation to the vessel and or the well. When the riser string is pressurized the rig heave motion vs. work over riser is normally compensated via a top drive heave compensation system and the possible telescopic element could either be moved to an end stop and or possibly locked, so that is may cope with the pressure within the riser string. There have previously been proposed a telescopic riser joint which will be able to handle pressures within the joint while at the same time allowing telescoping motion, for instance described in NO 169027. There are also telescopic joints which allow pressurized fluid within the telescope joint and actively control the upper part of the telescopic joint relative the vessel, for instance in the applicants own patent NO322172.
Having a telescopic joint which allow for pressure in the joint puts large demands on the seals in the system and control systems around the joint. This is the result of the present standard operations when the surface blow out preventer (SBOP) is located on top of the riser string, above the telescopic joint. Having the SBOP on deck also give rise to the issue of having an outlet for well fluids at high pressures, where this outlet also will be exposed for the end cap effect from the well at a deck on the vessel. This results in a situation which possibly is hazardous for personnel working in the vessel in case an accident as for instance a need for a quick release from the well.
An aim with the present invention is to form a riser system which improves HSE (health, security and environment) at the platform.
This is achieved with a riser system according to the following claims, where embodiments are given in the independent claims.